Contamination of the atmosphere by oxides of sulfur (SO.sub.x) and oxides of nitrogen (NO.sub.x) has been a public health and environmental problem for many years, because of the irritating effect on the human respiratory system, the adverse effect on plant life, and corrosive attack on many metals, fabrics, and building materials. These pollutants are generated from the combustion of sulfur and nitrogen compounds in the fossil fuels, such as oil and coal, and from the high temperature reaction of nitrogen and oxygen gases at the point of combustion, and are subsequently emitted as power plant flue gas, incinerator off-gas, and other waste gas from combustion engine, metallurgical plants, fertilizer plants and chemical plants.
One prior solution to this problem is to utilize wet scrubbing processes which involve washing the flue gas with various scrubbing solutions. Wet scrubbing processes can be effective in removing sulfur dioxide (SO.sub.2) and nitrogen dioxide (NO.sub.2) from the flue gases. However, nitric oxide (NO) is usually the major NO.sub.x species in waste gases from combustion processes. Due to the extremely low water solubility of nitric oxide (NO), the wet scrubbing processes are ineffective in removing nitric oxide (NO), unless the nitric oxide (NO) is oxidized to nitrogen dioxide (NO.sub.2). Gas phase oxidation of NO requires expensive oxidizers such as ozone or chlorine. Reaction-driven scrubbing processes utilizing chelated ferrous salts (e.g. EDTA-Fe (II) solution) to form a covalent Fe-NO bond with nitric oxide have been proposed. However, the presence of 3-5% oxygen in most waste gases tends to oxidize Fe(II) to Fe(III) and thus render these reaction-driven scrubbing processes ineffective unless a reducing agent is also provided. This approach is again economically not viable, unless Fe(II) can be practically regenerated, because the costs of the chemicals used are relatively high.
Another prior art solution to this problem comprises Selective Catalytic Reduction (SCR) of NO.sub.x to nitrogen (N.sub.2) with a catalyst and a reducing agent such as urea or ammonia. However, this method is expensive and also ineffective in removing SO.sub.x.
One other solution to this problem was proposed in Japanese Patent Application No. 54,026,966 published Feb. 28, 1989. This method comprises contacting the waste stack gas with a solution containing ferric nitrate and ferric sulfate. SO.sub.x is oxidized and dissolved as FeSO.sub.4 and NO.sub.x is oxidized to Fe(NO.sub.3).sub.2 OH. The drawback for this process is the poor removal of NO.sub.x, since ferric nitrate cannot effectively convert NO to NO.sub.2 which is more water soluble.
Still another solution to this problem comprises treating waste gas with manganese dioxide (MnO.sub.2) to form nitrate (NO.sub.3.sup.-) and sulfate (SO.sub.4.sup.=), while the manganese (IV) dioxide is reduced to manganous (II) compounds.
There are several patents disclosing the use of manganese dioxide for waste gas treatment. Chinese patent application No. 1,052,260 published Jun. 19, 1991 discloses a process for treating SO.sub.x using a soft Manganese ore containing about 60% MnO.sub.2. The Mn(IV) is reduced to Mn(II) and MnSO.sub.4 is recovered as a byproduct. No manganese regeneration is mentioned. A similar process is disclosed in Japanese patent application No. 3,207,427, published Sep. 10, 1991.
German Patent Application No. 3,731,899, published Apr. 27, 1989, discloses a stepwise removal of NO.sub.x which involves first converting NO to NO.sub.2 and subsequently oxidizing NO.sub.2 to manganous nitrate by MnO.sub.2. Thermal regeneration of MnO.sub.2 is mentioned.
U.S. Pat. No. 4,164,545, issued Aug. 14, 1979, described a process for treating waste gas utilizing MnO.sub.2 absorbent for NO.sub.x and SO.sub.x removal. The manganous ion formed is then recovered by ion exchange in acidic condition followed by thermal regeneration. This method is again tedious and expensive.
A flue gas wet scrubbing process using MnO.sub.2, KMnO.sub.4, FeS, and CuO and surfactant as scrubbing medium is described in Chinese Patent Application No. 87,107,725, published May 18, 1988. No regeneration of Manganese is mentioned.
Japanese Patent Application No. 51,136,592 and 51,136,595, both published on Nov. 26, 1976, disclose an anodic deposition method for the regeneration of a MnO.sub.2 -based catalyst. MnO.sub.2 is supported. Complicated dialysis and ion exchange procedures are involved.
U.S. Pat. No. 4,925,569, issued May 15, 1990, discloses a process for reducing the level of sulfides in aqueous solutions and gaseous in the presence of manganese-containing catalyst. There is no mention of removal of nitrogen oxides and sulfur oxides in the reference. There is again no mention of regeneration of manganese oxide.
These prior art processes are either tedious and/or too expensive. Those processes which utilize MnO.sub.2 are costly because they either consume large quantities of manganese compounds which are expensive, or the methods for regeneration of the manganese dioxide involve complicated and costly steps, such as thermal regeneration, ion exchange, etc. Since the 1990 Amendment to the Clean Air Act requires more facilities to have NO.sub.x control in addition to SO.sub.x control, there is a need for an improved process which can efficiently remove both NO.sub.x and SO.sub.x simultaneously and with a simple and economic method for the regeneration of the chemicals used.